Apparatus for spectroscopic analysis with modulated electrodeless discharge tube

ABSTRACT

A gas, or electrodeless, discharge lamp for use in spectroscopic analysis is excited in a microwave field of sufficiently high intensity to maintain a high-residual ion concentration within said lamp to reestablish the discharge therein with minimal delay. Also, the microwave field exciting the lamp is modulated, whereby the emission, or beam, from the lamp irradiating an atomic vapor is modulated in &#39;&#39;&#39;&#39;on-off&#39;&#39;&#39;&#39; fashion without physical obstruction of said beam.

United States Patent Dagnall 5] Feb. 29, 1972 [54] APPARATUS FOR SPECTROSCOPIC ANALYSIS WITH MODULATED ELECTRODELESS DISCHARGE TUBE [72] Inventor: Roy Maurice Dagnall, Chalvey, Slough,

[2]] Appl. No.: 838,975

[52] US. Cl. ..356/85, 250/226, 356/87 [5 1] Int. Cl. ..G0lj 3/30 [58] Field ofSearch ..356/85, 87; 250/l99 [56] References Cited UNITED STATES PATENTS 2,847,899 8/1958 Walsh ..356/85 OTHER PUBLlCATlONS Studies in Atomic-Fluorescence Spectroscopy Vlll" Browner, Dagnall and West; Talanta Vol. 16; Jan. I969; pg. 75 & 78

Primary Examiner-William L. Sikes Assistant Examiner-V. P. McGraw An0rneyS. P. Tedesco [57] ABSTRACT A gas, or electrodeless, discharge lamp for use in spectroscopic analysis is excited in a microwave field of sufficiently high intensity to maintain a high-residual ion concentration within said lamp to reestablish the discharge therein with minimal delay. Also, the microwave field exciting the lamp is modulated, whereby the emission, or beam, from the lamp irradiating an atomic vapor is modulated in on-off" fashion without 323:1 neat al. physicalobstruaion f i bear 3,386,33l 6/1968 Keller ..356/85 11 Claims,2Drawing Figures RESOVANT c4 r -4Q e l v 5 2 DISCHARGE 17-- TUBE 5 MICROWAVE GENERATOR 8wolvocmamron MODULATION 6 72 u/v/r 7 J PHO TOMUL T/PL IER RECORDER AMPLIFIER PATENTEDFEB29 I972 mac/MR6? rues MICROWAVE GENE RA TOR MODULA T/O/V 7 PHO TOMUL T/PL IE1? 9 AMPLIFIER RECORDER f FIG. 2.

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3a 4 13 Z @L@ Pg! AlMP. 2a a 80 76 APPARATUS FOR SPECTROSCOPIC ANALYSIS WITH MODULATED ELECTRODELESS DISCHARGE TUBE The invention relates to methods and apparatus for use in spectroscopic analysis by atomic absorption and fluorescence techniques.

In United Kingdom Pat. No. 763,556, there are described a method and apparatus for carrying out spectroscopic analysis by atomic absorption. In the method described, an atomic vapor has passed therethrough a beam of radiation the spectrum of which contains an atomic spectral line characteristic of the element of isotope, the concentration of which in the vapor is to be determined. The radiation passing through the vapor is detected and used as a basis for determining the concentration of the substance in the vapor.

The atomic vapor is formed by spraying a solution of the material to be analyzed into a high-temperature flame and it is, therefore necessary to provide means for eliminating the effects of radiation emitted by the vapor itself. In the construction described in the above-numbered patent, this elimination is effected by positioning a mechanical chopper between the radiation source and the flame to provide a modulation. The detection apparatus is designed to respond to a radiation input corresponding to the modulation frequency of the beam, but not to unmodulated signals.

In atomic adsorption spectroscopy, hollow cathode lamps, for example of the kinds disclosed in United Kingdom Pat. Nos. 896,744, 1,019,845 and 1,051,920, have until the present been used almost exclusively. Although generally satisfactory, these lamps possess a number of undesirable features.

Such lamps tend to suffer from self-reversal and only have a relatively short shelf life. They are expensive, which is an especially significant factor where a large number of elements need to be examined which require a corresponding number of lamps. in addition, they require a warmup period before use, which makes them unsatisfactory for use in automatic analysis procedures where spectrochemical analysis may comprise only one stage in a series of analytical stages which receive a sample for analysis sequentially one after another. For some elements, hollow cathode lamps are difficult to produce and for others the working range is insufficiently wide or the intensity is too low. Furthermore, the analytical signal is subject to a degree of distortion due to impurities in the emission resulting from the presence of the carrier gas spectrum together with ion lines and also lines of extraneous elements present in the lamp electrodes.

in addition, although adequate for absorption procedures, hollow cathode lamps are generally of an insufficiently high intensity for use in fluorescence spectroscopy. In the fluorescence process, the analytical signal varies linearly with the source intensity, and the capacity to detect low concentrations of a substance, therefore increases directly with the intensity of the spectral source.

These general considerations have led to the development of electrodeless discharge lamps for use both in atomic absorption and in atomic fluorescence procedures and a method of producing such lamps is described in the paper by Mr. R. M. Dagnall, K. C. Thomson and T. S. West in Talanta 1967 Vol. 14 pp. 551 to 555. Lamps prepared according to this method are between and 800 times more intense at the principal resonance lines than the comparable hollow cathode lamps.

The advantages accruing in terms of sensitivity from the use of electrodeless discharge lamps, however have been to same extent nullified by the use of mechanical choppers, for example, as mentioned in United Kingdom Pat. No. 763,556, for effecting modulation. The chopper has the effect both of physically spacing the light source from the atomic vapor and of preventing a considerable proportion of the light from reaching the flame. This situation, although undesirable in absorption procedures, is unacceptable in fluorescence techniques where the analytical signal from the flame is of very low intensity. As stated above, the analytical signal varies with source intensity and the use of a chopper consequently results in a reduction in the intensity of the analytical signal and a substantial reduction in the sensitivity of the apparatus.

It is among the objects of the invention to provide means for use in atomic absorption and fluorescence spectroscopy whereby the emission from an electrodeless discharge lamp may be modulated without physical obstruction of the emitted beam.

Although prior to this invention, electronic modulation of hollow cathode lamps has been effected, it was not thought possible to electronically modulate electrodeless discharge lamps due to the delay time required for initiating ionization throughout the lamp. However, providing the modulation frequency is sufficiently high, the residual ions in the tube are apparently sufficient to reestablish the discharge without delay.

Two arrangements incorporating the invention will now be described with reference to the accompanying drawings in which:

FIG. 1 is a semiperspective block diagram of an apparatus for effecting spectroscopic analysis by utilization of an atomic fluorescence emission, and

FIG. 2 is a block diagram of an apparatus for effecting spectroscopic analysis by atomic absorption.

Referring first to HO. 1, a resonant cavity 1 is shown, in which an electrodeless discharge tube 2 is mounted for excitation. The cavity may be of a Broida, Evenson or other type, dependent upon the nature or combination of the elements present in the tube.

The tube 2 is shielded so as to emit a light beam 3, when excited, towards a flame 4 which acts as a carrier for an atomic vapor and formed above a burner head 5.

The fluorescence emission from the flame (which constitutes the analytical signal) is detected along a line 6 at an angle to the light beam 3 by a photomultiplier 7 after passage through a monochromator 8. The signal from the photomultiplier 7 is fed to an AC amplifier 9 and the amplified signal recorded on a recorder I0. The amplifier 9 may be similar to that incorporated in any conventional atomic absorption spectrometer and includes the usual internal electronic chopper.

The resonant cavity 1 is energized from a microwave generator 11 the output of which has a superimposed modulation derived from a modulator unit 12. The unit 12 is also connected to the amplifier 9 and phases the signal amplification with the output of the generator.

The microwave generator II may comprise any commercial unit at present in use for medical diathermy or a purpose built unit such as the Microtron 200 generator manufactured by Electro-Medical Supplies Limited 209 B Great Portland Street, London W. 1. England. Similarly, any suitable microwave modulation unit may be used as the unit 12, the Microtron modulator unit Mark II, also manufactured by the above-mentioned company being one instrument suitable for the purpose.

The microwave generator 11 comprises a magnetron, the power output of which is normally controlled by varying the anode voltage. The modulation unit comprises a modulation transformer which superimposes a modulation signal 50 Hz. modulation has been found to be suitable-on the DC potential of the anode. The primary of the modulation transformer is supplied from mains supply through a variable transformer.

The DC supply of the magnetron incorporates a smoothing circuit and in order to prevent this from shunting the modulation signal, a choke and a capacitor are incorporated in the anode circuit of the magnetron to form an isolating filter.

The modulation unit 12 also supplies a low-voltage AC signal and has a bridge network, whereby the phase relationship of this signal to the mains supply may be altered. The output from the bridge network is fed via a power amplifier to the electronic chopper of the AC amplifier 9 so as to correctly phase the detector with the output of the discharge tube.

Turning now to FIG. 2, the components shown are substantially identical to those described above with reference to FIG. I but rearranged for absorption spectroscopy. For convenience the same reference numerals are used in this embodiment for the corresponding components but are here suffixed a.

in this arrangement, the monochromator 8a is disposed in alignment with the light beam 3a from the electrodeless discharge tube 2a. The radiation 13 resulting from absorption is received by the photomultiplier 7a after passage through the monochromator 8a and then amplified and recorded by the amplifier 9a and recorder 10a as before. Also as before the resonant cavity In is energized by a microwave generator 11a the output of which is modulated by a modulator unit 12a. This modulator unit also serves to phase the amplifier 9a to the output of the discharge tube 2a.

What we claim is:

l. A method for effecting spectroscopic analysis comprising the steps of: developing an atomic vapor containing a material whose concentration is to be determined, generating and exciting said vapor with a beam of electromagnetic radiation containing a spectral line characteristic of a material contained in said vapor, said beam being generated by positioning an electrodeless discharge lamp in a microwave field, modulating said microwave field with a superimposed modulation to modulate said beam in on-off fashion, the frequency of said superimposed modulation being sufficiently high to maintain sufficient residual ions within said tube to initiate ionization therein without substantial delay, irradiating said vapor with said modulated beam without physically obstructing said beam, and detecting the emission from said vapor to determine the concentration of said material.

2. The method of claim 1 further comprising the steps of detecting an emission from said vapor consequent upon irradiation of said vapor, and deriving an electrical signal therefrom, amplifying the electrical signal in phase with said superimposed modulation, and recording the amplified signal to indicate the concentration of said material in said vapor.

3. The method of claim 2, further comprising the steps of detecting the emission from said vapor in alignment with the modulated beam for carrying out analysis by atomic absorption spectroscopy.

4. The method of claim 2, further comprising the steps of detecting the emission from said vapor at an angle to the modulated beam for carrying out analysis by atomic fluorescence spectroscopy.

5. The method of claim 4, further comprising the step of detecting the emission from said vapor at to the modulated beam.

6. A device for spectroscopic analysis comprising: means for developing an atomic vapor containing a material whose concentration is to be determined, an electrodeless discharge tube, means for positioning said tube in a microwave field so as to excite said tube and produce a beam of electromagnetic radiation having a spectral line characteristic of said material, said tube when excited being positioned to irradiate said vapor, means for modulating said microwave field with a superimposed modulation of sufficient amplitude to modulate said beam irradiating said vapor in on-off fashion without physical obstruction of said beam, the frequency of said superimposed modulation being sufficiently high to maintain sufficient residual ions within said tube to initiate ionization therein without substantial delay, and means for detecting emission from said vapor to determine the concentration of said material.

7. An apparatus as claimed in claim 6, further comprising means responsive to said detecting means for deriving an electrical signal therefrom, means for amplifying the electrical signal in phase with the modulation of the beam, and means for recording said electrical signal to indicate the concentration of said material.

8. An apparatus as claimed in claim 7, wherein said detecting means comprises a monochromator and photomultiplier sequentially aligned with said tube and said developing means for determining the concentration of said material by atomic absorption spectroscopy 9. n apparatus as c aimed in claim 7, wherein said detecting means comprises a monochromator and photomultiplier sequentially aligned on a line at an angle to the line between said tube and said developing means for determining the concentration of said material by atom fluorescence spectrosco- 10. An apparatus as claimed in claim 6 wherein said exciting means comprises a resonant cavity for receiving said tube and a microwave generator connected to said cavity for producing said microwave field to excite said tube, and means for modulating the power output from said generator to produce a corresponding modulation in said microwave field and in said beam produced by said tube.

11. An apparatus as claimed in claim 10, wherein said generator is biased at a predetermined DC potential. 

1. A method for effecting spectroscopic analysis comprising the steps of: developing an atomic vapor containing a material whose concentration is to be determined, generating and exciting said vapor with a beam of electromagnetic radiation containing a spectral line characteristic of a material contained in said vapor, said beam being generated by positioning an electrodeless discharge lamp in a microwave field, modulating said microwave field with a superimposed modulation to modulate said beam in onoff fashion, the frequency of said superimposed modulation being sufficiently high to maintain sufficient residual ions within said tube to initiate ionization therein without substantial delay, irradiating said vapor with said modulated beam without physically obstructing said beam, and detecting the emission from said vapor to determine the concentration of said material.
 2. The method of claim 1 further comprising the steps of detecting an emission from said vapor consequent upon irradiation of said vapor, and deriving an electrical signal therefrom, amplifying the electrical signal in phase with said superimposed modulation, and recording the amplified signal to indicate the concentration of said material in said vapor.
 3. The method of claim 2, further comprising the steps of detecting the emission from said vapor in alignment with the modulated beam for carrying out analysis by atomic absorption spectroscopy.
 4. The method of claim 2, further comprising the steps of detecting the emission from said vapor at an angle to the modulated beam for carrying out analysis by atomic fluorescence spectroscopy.
 5. The method of claim 4, further comprising the step of detecting the emission From said vapor at 90* to the modulated beam.
 6. A device for spectroscopic analysis comprising: means for developing an atomic vapor containing a material whose concentration is to be determined, an electrodeless discharge tube, means for positioning said tube in a microwave field so as to excite said tube and produce a beam of electromagnetic radiation having a spectral line characteristic of said material, said tube when excited being positioned to irradiate said vapor, means for modulating said microwave field with a superimposed modulation of sufficient amplitude to modulate said beam irradiating said vapor in on-off fashion without physical obstruction of said beam, the frequency of said superimposed modulation being sufficiently high to maintain sufficient residual ions within said tube to initiate ionization therein without substantial delay, and means for detecting emission from said vapor to determine the concentration of said material.
 7. An apparatus as claimed in claim 6, further comprising means responsive to said detecting means for deriving an electrical signal therefrom, means for amplifying the electrical signal in phase with the modulation of the beam, and means for recording said electrical signal to indicate the concentration of said material.
 8. An apparatus as claimed in claim 7, wherein said detecting means comprises a monochromator and photomultiplier sequentially aligned with said tube and said developing means for determining the concentration of said material by atomic absorption spectroscopy.
 9. An apparatus as claimed in claim 7, wherein said detecting means comprises a monochromator and photomultiplier sequentially aligned on a line at an angle to the line between said tube and said developing means for determining the concentration of said material by atom fluorescence spectroscopy.
 10. An apparatus as claimed in claim 6 wherein said exciting means comprises a resonant cavity for receiving said tube and a microwave generator connected to said cavity for producing said microwave field to excite said tube, and means for modulating the power output from said generator to produce a corresponding modulation in said microwave field and in said beam produced by said tube.
 11. An apparatus as claimed in claim 10, wherein said generator is biased at a predetermined DC potential. 